This invention relates generally to implantable defibrillators and more particularly to a method and apparatus for providing more efficient ventricular defibrillation shocks.
Cardiac arrhythmias can generally be thought of as disturbances of the normal rhythm of the heart muscle. Cardiac arrhythmias are broadly divided into two major categories, bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia can be broadly defined as an abnormally rapid heart (e.g., over 100 beats/minute, at rest), and bradyarrhythmia can be broadly defined as an abnormally slow heart (e.g., less than 50 beats/minute). Tachyarrhythmias are further subdivided into two major sub-categories, namely, tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart are rapid, but organized. Fibrillation is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized. Tachycardia and fibrillation are further classified according to their location within the heart, namely, either atrial or ventricular. In general, atrial arrhythmias are non-life threatening, chronic conditions, because the atria (upper chambers of the heart) are only responsible for aiding the movement of blood into the ventricles (lower chambers of the heart), whereas ventricular arrhythmias are life-threatening, acute events, because the heart's ability to pump blood to the rest of the body is impaired if the ventricles become arrhythmic. This invention is particularly concerned with treatment of ventricular fibrillation.
Since an individual who experiences fibrillation typically will not always be immediately accessible by emergency care technicians and their equipment, and/or will become incapacitated and unable to beckon such care, implantable cardiac stimulation devices have become critical delivery systems of emergency care for many patients with chronic heart failure problems.
Various types of implantable cardiac stimulation devices are presently available and used for delivering various types of cardiac stimulation therapy in the treatment of cardiac arrhythmias. The two most common types which are in widespread use are pacemakers and implantable cardioverter defibrillators (ICDs).
Pacemakers generally produce relatively low voltage pacing pulses which are delivered to the patient's heart through low voltage, bipolar pacing leads, generally across spaced apart ring and tip electrodes thereof which are of opposite polarity. These pacing pulses assist the natural pacing function of the heart in order to prevent bradycardia.
On the other hand, ICDs are sophisticated medical devices which are surgically implanted (abdominally or pectorally) in a patient to monitor the cardiac activity of the patient's heart, and to deliver electrical stimulation as required to correct cardiac arrhythmias which occur due to disturbances in the normal pattern of electrical conduction within the heart muscle. In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrical signals, known as electrograms (EGMs), detected by sensing electrodes positioned in the patient's heart. More particularly, contemporary ICDs include waveform digitization circuitry which digitizes the analog EGM produced by the sensing electrodes, and a microprocessor and associated peripheral integrated circuits (ICs) which analyze the digitized EGM in accordance with a diagnostic algorithm implemented by software stored in the microprocessor. Contemporary ICDs are generally capable of diagnosing the various types of cardiac arrhythmias discussed above, and then delivering the appropriate electrical stimulation/therapy to the patient's heart, in accordance with a therapy delivery algorithm also implemented in software stored in the microprocessor, to thereby correct or terminate the diagnosed arrhythmias. Typical electrical stimulus delivery means used in ICDs involve an energy storage device, e.g., a capacitor, connected to a shock delivering electrode or electrodes. Contemporary ICDs are capable of delivering various types or levels of electrical therapy. U.S. Pat. No. 5,545,189 provides a representative background discussion of these and other details of conventional ICDs, and the disclosure of this patent is herein incorporated by reference.
One conventional method of electrical shock therapy for treating ventricular arrhythmia is to deliver a single burst of a relatively large amount of electrical current through the fibrillating heart of a patient by an ICD supported-electrode configuration installed in or about the patient's heart. For a given ventricular fibrillation episode, the minimum amount of energy required to defibrillate a patient's ventricle is known as the ventricular defibrillation threshold (VDFT). However, in the treatment of an acute cardiac condition, such as ventricular fibrillation, conventional ICD-based therapies have encountered a dilemma in that while higher strength defibrillation shocks generally have a higher probability of success of achieving defibrillation than lower strength shocks, the countervailing consideration is that higher energy shocks demand commensurately greater ICD equipment capabilities and cost, such as in terms of batteries, capacitors, and so forth.
It has been experimentally observed that the likelihood of successful defibrillation has been shown to follow a sigmoidal shaped curve in which higher strength shocks have a higher probability of success than lower strength shocks. See, e.g., Davy J., et al., "The relationship between successful defibrillation and delivered energy in open-chest dogs: reappraisal of the "defibrillation threshold" concept," Am Heart J. 1987;113:77-84. When a number of shocks are applied at the V50 level, 50% of applied shocks are expected to result in successful defibrillation. In order to interpret the increased probability of success in terms of percentage improvement in DFT, some previously published data is available to illuminate the issue. For a superior vena cava (SVC) lead and right ventricle (RV) lead configuration, for example, the probability of success curves have been developed to determine that (V80-V50)/V50=0.14. E.g., see Souza et al., "Comparison of upper limit of vulnerability and defibrillation probability of success curves using a nonthoracotomy lead system," Circulation, 1995, 91:1247-1252. By linear approximation of the central portion of the sigmoidal curve, this yields (V70-V50)/V50=0.09. This equation indicates that if the probability of success in achieving defibrillation at a certain voltage is 50%, then increasing the voltage by 9% will increase the probability of success to 70%.
Yet, the ICD device preferably should be designed to be as small in dimensions and light in mass as possible so as to be less cumbersome and bulky to the patient, so it generally will not be practical to significantly scale-up the power and voltage capabilities of an ICD device in many cases as the mode of increasing the probability of success in the delivery of defibrillation therapy.
Instead, it would be desirable to find ways to lower the VDFT for a given ICD size and power. Furthermore, a patient having an installed ICD may experience several or more acute separate fibrillation episodes a year requiring intervention by the installed ICD unit. Thus, it can be appreciated how lowering of the energy requirements demanded of the ICD would be desirable so as to prevent premature depletion of the batteries, and thereby increase the service life of the ICD device.
Also, while a patient experiencing a ventricular fibrillation episode, may or may not be conscious or semi-conscious, it is still possible that the patient could potentially perceive any programmed electrical stimulation treatment being performed on his/her heart during the episode.
Thus, to mitigate any possible further trauma to the patient on account of any negative perceptions of the electrical jolts accompanying the VDF shocks, or, alternatively, to reduce the risk of inadvertent myocardial tissue damage from the delivered shock, it also would be desirable to reduce the ventricular defibrillation threshold (VDFT) for these additional reasons.
Thus, it is desirable for reasons of both increased device longevity and patient comfort/safety to reduce the amount of energy required to defibrillate a patient's heart when using an implantable cardioverter defibrillator (ICD). However, this goal had not previously been fully satisfied in the ICD field despite active interest and numerous experimental studies reported in the relevant field.
As generally known, during ventricular fibrillation (VF), it has been observed that shocks of the same voltage can at some times achieve successful defibrillation (DF), yet fail at other times. It has been observed that a factor contributing to this phenomena is that the electrophysiological state of cardiac tissue in the so-called "low gradient region" of the heart can exist in various conditions of depolarization and repolarization at various times.
The "low potential gradient region," or "low gradient region" for short, is that region of the heart tissue that is most remote from the defibrillation electrodes and thus experiences a lower electrical gradient relative to other portions of the heart at the time of delivery of a defibrillation shock. More specifically, the low potential gradient region thus is where the electric field lines generated by the current flowing between a pair of defibrillation electrodes positioned in the heart are the least densely spaced. The location of this region can vary to the extent that the potential gradients generated by a defibrillation shock depend upon the particular lead configuration of the defibrillation electrodes in the heart, the tissue conductivities, and torso geometry. The low potential gradient region can be located by measurement or intuitively.
Previous cardiac mapping studies have demonstrated that, following a failed defibrillation shock, the earliest sites of propagation from which post-shock activation wavefronts originate tend to appear from regions where cells are just emerging from their effective refractory period immediately prior to the defibrillation shock. See, e.g., Chen, P., et al., "Comparison of activation during ventricular fibrillation and following unsuccessful defibrillation shocks in open-chest dogs," Circ Res. 1990;66:1544-1560; Zhou, X., et al., "Epicardial mapping of ventricular defibrillation with monophasic and biphasic shocks in dogs," Circ Res. 1993;72:145-160; Walcott, G., et al., "Mechanisms of defibrillation for monophasic and biphasic waveforms," PACE. 1994;17:478-498.
Prior studies also have shown that these earliest sites of propagation are the regions where extracellular potential gradients are lower than a critical value. It follows from this that the shock strength in the low gradient region can be too low to cause any extension of refractoriness in the local tissue. Moreover, in the low gradient region, shocks can cause action potential stimulation only when they are applied very late in the repolarization phase. See, e.g., Swartz, J., et al., "The conditioning prepulse of biphasic defibrillator waveforms enhances refractoriness to fibrillation wavefronts," Circ Res. 1991;68:438-449; Dillon, S., et al., "Optical recordings in the rabbit heart show that defibrillation strength shocks prolong the duration of depolarization and the refractory period," Circ Res. 1991;69:842-856; Dillon, S., et al., "Synchronized repolarization after defibrillation shocks--A possible component of the defibrillation process demonstrated by optical recordings in rabbit heart," Circulation. 1992;85:1865-1878. These results have been interpreted to suggest that one possible factor contributing to the probabilistic character of defibrillation is that the state of repolarization of tissue in the low gradient region is different at different times.
Despite the improved understanding being developed in the field on the relationship of the electrophysiological characteristics of the low gradient region of the heart and efficacy of defibrillation therapy, there still exists a need for a modality of delivering cardiac therapy that improves the probability of success of a ventricular defibrillation shock while also reducing ventricular defibrillation thresholds (VDTs) to reduce energy demands placed upon an ICD device and to reduce the risk of pain, trauma or myocardial tissue damage to a patient undergoing defibrillation treatment.
The above and other objects, benefits and advantages are achieved by the present invention as described herein.